Think that just because you use JUnit and test-driven development that your code is well tested? Actually, probably not.

One of the key tenets of XP is that no line should be in your code that isn't necessary. If you take a line out of your code it should break at least one test. If it doesn't, delete it or write more tests!

So you could spend your time deleting lines and then running the test suite to see if it breaks ... but who wants to do that? That's where a code coverage tool like jcoverage comes in.

Coverage tools work by marking the compiled code so that lines can be counted by the number of times they are executed. Then you can run your test suite over this modified version of your compiled code to see where the test suite hits and where it misses (see picture). You can even see which specific lines of code you forgot to cover.

Then you have a choice: write the tests that cover those lines or decide they are useless and remove them. Some lines are impossible to get to -- a silly example would be:

if (x < 10) {
   y += 1;
   if (100 < x) {
      System.out.println("Am I getting here?");
   }
}

You would never get to the println(). When you refactor complicated code over and over it's hard to tell when you've cut a chunk of code off and made it impossible to execute. That cruft can be removed to simplify what's left.

So your tests test your code and coverage tools examine your tests. It's a great way to keep on top of the quality of your tests and your code. So get a code coverage tool! If you have the means I highly recommend picking one up ... it is so choice.